Thursday, April 21, 2016

The low pressure system on the weather maps today, both on the surface and aloft, has been taking its sweet time moving through the U.S. The upper level system moved into the Pacific Northwest a week ago (April 14), and this morning was located over Iowa.

500 millibar map for 7:00 a.m. CDT April 21

Over the last week this system wreaked havoc on the central Rockies with heavy snow, and in southern Texas with record heavy rain. The slow progression of the low was due to what is called and omega block in the upper atmosphere. That effectively parked the upper low over the Great Basin and Rockies for several days. As is typical in these situations this was a good news, bad news situation. The good news was sunny, warm spring weather over the eastern half of the country. The bad news was a heavy spring snowstorm in the Rockies, and record heavy rainfall and flooding in southern Texas, particularly in the Houston area.

500 millibar map for 7:00 a.m. CDT April 17.

With the upper low parked over the Rockies, southerly winds on the east side of the low funneled copious amounts of moisture northward through Texas and into the central Rockies and western Plains. Very cold air aloft, ample moisture, strong upward motion, and the fact that the system was barely moving resulted in an extended period of snow from Wyoming south through Colorado into northern New Mexico last weekend. By late Sunday more than 4 feet of snow - heavy, wet snow - had fallen in some locations on the east side of the front range above 9000 feet. Two to three feet was common in the Front Range foothills.

72 hour snowfall ending the morning of April 18.

4-day CoCoRaHS snow totals for locations in Colorado

Denver (Stapleton Coop site) picked up 8.4 inches of snow from the storm, but amounts varied from 6 to 12 inches across the metro area. This latest storm boosted Denver's snow for the season to 71.4 inches (11.4 inches for the storm and a season total of 69.3 inches at Denver International). That was enough to bump the Accumulated Winter Season Severity Index (AWSSI) for Denver back into the severe category for this year.

Far bigger problems were in store for Texas. A cold front trailing south from the surface low in Kansas stalled out and provided a focus for the development of heavy showers and thunderstorms. Southerly winds fed moisture laden air with dewpoints in the low 70s into southern Texas. This air collided with the colder, drier air north and west of the front and was forced upward, helping sustain heavy thunderstorms from Dallas south to Houston.

Surface map for 4:00 a.m. CDT April 18 for the southwest U.S.

The rain was heaviest in the Houston area, and was enhanced by an outflow boundary from thunderstorms that helped further sustain the rainfall. Thunderstorms regenerated and trained repeatedly over the same area. Houston's Intercontinental Airport set a one-day rainfall record of 9.92 inches on April 18, breaking the old record by almost two inches (8.16 inches in 1976). However, 10 inches was far from the highest amounts recorded. Those occurred in the northwest quadrant of the metro area. Measured rainfall amounts were in excess of 15 inches for the 24-hour period, and radar estimates were as high as 20 inches.

Quantitative Precipitation Estimate for southern Texas for the 24-hour period ending at 7:00 CDT April 18.

48-hour precipitation for the period ending 7:00 a.m. CDT April 19.Source: Harris County Flood Warning Service

A number of CoCoRaHS observers recorded 12 inches or more on the morning of April 18. Much of this fell in a 12 to 13 hour period.

Southern Texas CoCoRaHS observations on April 18.

Flooding was rapid and widespread. Despite warnings to stay off the streets and not drive into flooded roads, eight people lost their lives after being trapped in floodwaters. Unfortunately the rain didn't quit completely, and another two to three inches fell in the Houston area through this morning.

Total precipitation for the 72-hour period ending at 8:00 a.m. April 20

Flooding is going to remain a problem in Houston for a number of days. Two dams in the area are considered "extremely high risk" by officials and are being closely monitored. The reservoirs behind them are at about 80 percent capacity.

Houston flooding on April 18.Credit: Reed Timmer via Twitter

The rain wasn't limited to the Houston area. Five to eight inches of rain fell in the Dallas-Ft.Worth area resulting in flooding in northern Texas.

The pesky upper low responsible for this week of stormy weather will finally weaken and move out into the Atlantic late Saturday. Then, we'll turn our attention to the next system in the Pacific Northwest which may mean more snow for the Rockies and an unsettled week in the Plains and Midwest.

Wednesday, April 13, 2016

There's a saying that "everything is bigger in Texas", and this week Texans saw some big big hail. On both Monday and Tuesday severe thunderstorms pounded areas with large hail and strong winds, a very damaging combination. Insurance adjusters are going to be very busy in Texas for awhile.

On Monday, April 11 severe thunderstorms pounded the area around Wylie, TX, just east of Plano and northeast of Dallas, with 2 to 5 inch diameter hail.

The storm hit during the late afternoon/early evening and moved ESE through the region.

Winds gusted to 60-70 mph at times with these storms, and that turned golf-ball to baseball-size hail into destructive and deadly missiles. The hail produced extensive damage to vehicles that were outside, but also caused large amounts to homes as the wind-driven hail shattered windows, pockmarked siding, and even punched through roofs and ceilings. This wasn't just 30 seconds of hail - in many cases it lasted for several minutes.

Softball-size hail (4.5") in Wylie.Credit: Kristin Baxter via Twitter

Holes punched through a roof (left) and damage to an exterior wood door (right) from large hail in Wylie, TX. Source: Twitter

The damage was extensive enough that the Wylie school district canceled Tuesday classes at all 19 campuses. Damage occurred not only to school buses but to many of the school buildings as well.

This image is of a product named MESH (Maximum Expected Size of Hail) uses multiple radar data parameters to depict the swath of hail and the size expected.

Hail swath as of 6:50 p.m. CDT April 11, showing the largest hail over Wylie, TX

The Forth Worth office of the National Weather Service provided this 3-D depiction of the storm as the largest hail was falling on Wylie.

On Tuesday, April 12 the hail threat shifted into south Texas. Storms developing along the Rio Grande moved east dropping hail 2 to 3.5 inches in diameter.

Hail swaths associated with the severe thunderstorms in Texas on April 12. San Antonio is located in the center upper third of the image (labeled SAT).

The storm with the largest hail hit near and in San Antonio. Again, many vehicles were damaged, and one report indicated that one hundred to perhaps as many as 300 luxury vehicles were damaged at a BMW dealer lot in San Antonio.

Hail that fell on San Antonio on April 12.

This is not the first time this season that severe hail has pummeled Texas. On March 17 a storm rolled through the Fort Worth area dropping 1 to 2.5 inch hail, causing an estimated $300 million damage to vehicles alone. Damage estimates for Wylie, when finally compiled, could be staggering. Many vehicles were totaled and there was extensive damage to homes. The storm in San Antonio Tuesday may end up being the costliest on
record there, with preliminary damage estimates of $125 million. Texas hail events in March, along with these latest two storms are likely to push damages in the past month alone to in excess of $2 billion.

Measuring hail is a core mission of CoCoRaHS, and the separate hail reports on the CoCoRaHS web site allow you to submit your hail information. There are a few things you need to know before measuring hail, and you can find that information in our "Measuring Hail" training animation. Here is a hail size reference and measuring guide you can download, print, and laminate for use. You can download it here.